Basionym

A basionym is, in botanical and zoological nomenclature, the original name upon which a new name is based. It is the validly published name that serves as the nomenclatural type of a later name. In simpler terms, it is the original, "base" name that is carried over when a species or other taxon is reclassified, but the name itself changes to reflect the new classification. The basionym always retains its original authorship and publication details, even when the new name is used.

The concept is important for tracking the history and application of names, ensuring clarity and stability in scientific nomenclature. When a new combination is made, the basionym is explicitly cited, indicating the origin of the epithet. For instance, if species A is moved from genus X to genus Y, resulting in the name Y something (where something is derived from the original name of species A), then the basionym would be X something, along with its original author(s) and year of publication. Citing the basionym provides a clear link to the previous name and prevents ambiguity in identifying the organism in question.

The basionym rule applies to names at any rank, including species, genera, and families. It is a fundamental principle governed by the International Code of Nomenclature for algae, fungi, and plants (ICNafp) and the International Code of Zoological Nomenclature (ICZN).